The present invention relates generally to wireless charging systems and, more particularly, to techniques for detecting the presence of foreign objects in wireless charging systems.
In a conventional wireless charging system, a power source (referred to herein as a power-transmitting node or TX) transmits power wirelessly via inductive coupling to a power sink (referred to herein as a power-receiving node or RX) that is placed on or at least near the TX in order to charge or power the RX. The inductive coupling between a TX and an RX is achieved via resonant transducer circuitry in each node having similar if not identical resonant frequencies. To determine whether an RX is present, the TX will periodically or intermittently execute digital pings and, if present, an RX will respond by transmitting an ack message acknowledging its presence. The term “digital ping” refers to the TX inserting energy into its resonant transducer circuitry, which thereby transfers power to the RX's resonant transducer circuitry, which in turn causes the RX to transmit an ack message back to the TX. After receiving the ack message from the RX, the TX will initiate a power-transfer session to transmit power to the RX. During a power-transfer session, the RX will transmit CEP (control error packet) messages instructing the TX to increase or decrease its transmitted power level.
If a metal foreign objected (FO), like a coin, a key, or other metal object, is placed on or at least near the TX during a power-transfer session, inductive coupling between the TX and the FO may result in the generation of heat in the FO that can be a fire hazard or result in damage to the FO, the TX, and/or the RX. As such, the conventional TX is designed to monitor power loss during its power-transfer sessions, where power loss is defined as the amount of power transmitted by the TX that is not received by the RX.
To enable a TX to determine power loss, a conventional RX monitors its level of received power and periodically or intermittently transmits RP (received power) packets to the TX informing the TX of the RX's received power level. The TX monitors its level of transmitted power and determines the power loss as the difference between the TX's transmitted power level and the RX's received power level. If the TX determines that the power loss exceeds a specified power-loss threshold, then the TX determines that an FO is present, and in such case, the TX terminates the power-transfer session and enters a protection state in which the TX cannot transfer power to the RX.
The conventional RX determines its received power level by monitoring the current and voltage levels within its resonant transducer circuitry. Similarly, a conventional TX determines its transmitted power level by monitoring the current and voltage levels within its own resonant transducer circuitry. Since the RX and TX power levels vary over time, the TX will compare the current value of the RX's received power level with the current value of its own transmitted power level that corresponds to the same time period during which the RX measured its received power level.
When a CEP or RP packet is transferred from an RX to a TX during a power-transfer session, the current and voltage levels in the resonant transducer circuitries of the RX and the TX can be disrupted enough such that the respective determined received and transmitted power levels may be inaccurate. As such, the RX and the TX do not measure their respective received and transmitted power levels during periods of packet transfer.
FIG. 1 is a timing diagram representing processing implemented at the conventional RX and the conventional TX associated with the TX's detection of the presence of an FO. FIG. 1 represents an example time period during which the RX transmits two successive CEP packets followed by an RP packet. As indicated, the RX accumulates data corresponding to its received power level during time periods between transmissions of packets to the TX. The RX knows when to stop and start accumulation of its received power level data based on the known beginning and end of its transmission of each packet to the TX. Similarly, the TX accumulates data corresponding to its transmitted power level during periods between receipt of packets from the RX. The TX stops accumulating its transmitted power level data when the TX detects the beginning of each received packet and resumes data accumulation when the TX detects the end of each received packet.
For example, at the end of transmission of CEP packet #1, the RX resumes accumulating data corresponding to its received power level. Similarly, at the end of receipt of CEP packet #1, the TX resumes accumulating data corresponding to its transmitted power level.
At the beginning of transmission of CEP packet #2, the RX stops accumulating data corresponding to its received power level. Similarly, at the beginning of receipt of CEP packet #2, the TX stops accumulating data corresponding to its transmitted power level.
At the end of transmission of CEP packet #2, the RX resumes accumulating data corresponding to its received power level. Similarly, at the end of receipt of CEP packet #2, the TX resumes accumulating data corresponding to its transmitted power level.
At the beginning of transmission of the RP packet, the RX stops accumulating data corresponding to its received power level. Similarly, at the beginning of receipt of the RP packet, the TX stops accumulating data corresponding to its transmitted power level.
The RX's received power level information contained in the RP packet is generated by the RX based on a subset of its current accumulation of data corresponding to its received power level. In particular, as indicated in FIG. 1, the subset of data used by the RX corresponds to a timing window (whose duration is specified by the parameter “WINSIZE”) that ends an offset time period (whose duration is specified by the parameter “OFFSET”) before the beginning of transmission of the RP packet by the RX. Similarly, the TX generates its transmitted power level based on a subset of its current accumulation of data corresponding to its transmitted power level. In particular, as indicated in FIG. 1, the subset of data used by the TX corresponds to a timing window that is determined based on the same WINSIZE and OFFSET parameters relative to the beginning of receipt of the RP packet at the TX. Since the beginning of transmission of the RP packet by the RX substantially coincides with the beginning of receipt of the RP packet at the TX, the two timing windows are essentially identical. The TX then compares the RX's received power level to its own transmitted power level to detect whether an FO is present. Since the RX's received power level corresponds to the same time period as the TX's transmitted power level, the comparison provides a good indication of the presence or absence of a FO.
Note that the received and transmitted power levels are generated based on subsets of data accumulated just prior to the most-recent packet without including any data accumulated prior to any previously received packets. For example, in the example of FIG. 1, the data accumulated prior to CEP packet #2 is not used to generate the received and transmitted power levels by the RX and TX, respectively, after detecting the beginnings of transmission and receipt of the RP packet. Only data accumulated after CEP packet #2 is used.
There are times when a TX does not properly receive a packet transmitted from an RX. In that case, the RX's most-recent accumulated data will not correspond in time with the TX's most-recent accumulated data, so as described further below with respect to FIG. 2, with power levels that fluctuate over time, the TX's transmitted power level will be based on obsolete data, which can lead to false positive FO detection by the TX.
FIG. 2 is a timing diagram representing processing implemented at a conventional RX and a conventional TX associated with the TX's detection of a FO when a CEP packet is not properly received. The beginning of the timing diagram of FIG. 2 is the same as that of FIG. 1. In FIG. 2, however, the TX detects the start of CEP packet #2, but does not properly detect the end of that packet. As a result, the TX stops accumulation of its own transmitted power data when it detects the beginning of CEP packet #2, but the TX does not resume data accumulation at the end of CEP packet #2, because the TX fails to decode the CEP packet #2, and hence fails to detect the end of CEP packet #2. As a result, when the TX receives the RP packet from the RX, the information about the RX's received power level in the RP packet will be based on accumulated data within the appropriate timing window between the end of CEP packet #2 and the beginning of the RP packet, while the TX's transmitted power level will be based on accumulated data within the timing window between the end of CEP packet #1 and the beginning of CEP packet #2. Such a situation causes the TX to calculate an obsolete value for its transmitted power level, which can result in the TX generating a false positive detection of an FO. It would be advantageous to have a more-accurate power level calculation circuit in TX and RX nodes.